Brain functional imaging consists of imaging the transient changes of an area of the brain related to a neuronal activity, in animals or in humans. The areas of the brain corresponding to different specific functions (for example displacement of certain limbs, the use of certain sensorial detectors, emotions, memory, etc) are listed in known functional anatomic atlases, respectively corresponding to various animal species or to human beings (for example the “Paxinos” atlases), and brain functional imaging gives the possibility of locating and measuring the neuronal activity in certain of these functional areas, matching with a certain activity of the human or the animal on which is practiced functional imaging.
Certain brain functional imaging techniques are based on the measurement of electric currents (EEG) or magnetic fields (MEG) generated by the neurones. These techniques have a very limited spatial resolution because of the complexity of the inverse problem to be solved for again finding the position of the electric activity sources.
The other techniques giving the possibility of imaging these functional changes are based on the neuro-vascular coupling: when the neurones have strong activity in an area of the brain, a supply of glucose is required in this area. For this, the vascular flow increases in this specific area. By imaging the vascular flows in the brain, it is possible to infer therefrom which are the functional areas activated in the functional atlas of the human or the animal, the brain of which is imaged.
Several techniques may be used for imaging these changes in blood flow:                functional MRI (also called “MRIf”) which images the change in oxygenation related to a change in blood flow,        nuclear imaging (PET) which images the binding of glucose brought by the blood flow,        and the ultrasonic functional imaging.        
In fMRI, the localization of the activated functional area is easily accomplished, since the MRI images are of very good quality and give the possibility of quite easily locating the anatomy and therefore the activated functional area of the brain. The functional imaging is then carried out and superimposed to an anatomic image produced by the MRIf machine before acquiring the functional image. This technology however has many drawbacks: the fMRI machines are very costly and cumbersome, and only provide a good spatial resolution with the expense of a significant reduction of the time resolution, which does not give the possibility of imaging transient phenomena in the brain (epilepsy fit for example).
PET has very poor spatial resolution. It gives an interesting functional piece of information, but once the activated area has been located, it is not possible to know to which anatomic or functional area of the brain it corresponds, so that it is necessary to combine a PET imaging machine with an MRI machine or a CT scanner in order to obtain a useable result. Such an imaging assembly is however extremely expensive.
The ultrasonic functional imaging is based on ultrasensitive imaging of the blood flow (Mace et al, “Functional ultrasound imaging of the brain: theory and basic principle”, IEEE Trans Ultrasonic Ferroelectric Freq. Control. 2013 March; 60(3): 492-506), for which the variation overtime gives access to the activated functional areas (Mace et al, “Functional ultrasound imaging of the brain”, Nature Methods, 8, 662-664, 2011).
Ultrasonic functional imaging gives the possibility of obtaining at a relatively low cost, an extremely accurate vascular image of the brain and the localization of the activated areas on this vascular image. A difficulty is however to be able to have the vascular image correspond with an anatomic image giving the possibility of locating the functional areas. Indeed, the anatomic image which may be acquired in echography by the ultrasonic functional imaging device, is of too low quality for allowing localization of the imaged functional areas.
Consequently, only an expert operator is capable of localizing the functional areas imaged in ultrasonic functional imaging: he/she has to locate with the naked eye in the vascular image, global structures of the brain which will allow him/her to position the image in the brain, and then attempt to find where is located the functional area which is of interest to him/her by organ of an atlas.